Method of making silica sols containing large particle size silica



Aprll 22, 1969 w. ALBRECHT 3,440,174

METHOD OF MAKING SILICA SOLS CONTAINING LARGE PARTICLE SIZE SILICA FiledApril 26, 1965 Fig.1

Grams of snhca per Mnlhle'ker of Sol Inventor William L-Albrechi:

United States Patent Office 3,440,174 Patented Apr. 22, 1969 US. Cl.252--313 6 Claims ABSTRACT OF THE DISCLOSURE A method of producingconcentrated aqueous stable silica sols which contain at least 35% byweight of large, spherical, nonaggregated silica particles having aweightaverage particle of 45-100 millimicrons. Acid sol is fed to analkaline heel using a controlled feed rate.

This application is a continuation-in-part of my copending applicationSer. No. 165,617, filed Jan. 11, 1962, now abandoned.

This invention relates to concentrated silica sols and method of makingsame. With more particularity, this invention is concerned with silicasols containing at least 35% by weight of silica in the form of large,discrete, spherical particles having a Weight-average particle diameterof from 45 to 100 millimicrons, and a novel process for making theabove.

Silica in the form of uniform colloidal dispersions in water and otherhydrophilic liquids, commonly classified as silica sols, is well-knownin the art. Particularly during the last decade, intensive research hasbeen directed toward improvement of these silica sols and extension ofthe scope of uses to which they may be applied. Commercial andindustrial demands are pressing for a stable siliceous product in theform of a uniform sol which has the additional quality of relativelyhigh concentration of the active ingredient, silica. The benefits ofsupplying such a product in which the active ingredient, silica, ispresent in large amounts are obvious. For example, shipping and handlingcosts are reduced in direct proportion to the silica concentration ofthe product. In addition, many processes involving the use of suchsilica sol are effected at a considerable savings when such concentratedsilica products are employed as starting reactants or modifiers.

While stability and relatively high concentration of silica are primeconsiderations in research directed toward the silica sol art, there isan additional growing demand for silica products having theaforementioned properties, which additionally contain silica particlesof a relatively large particle size. In many processes theefliectiveness is directly related to the particular size of the silicaparticles which must be uniformly and colloidally dispersed in theliquid hydrophilic carrier. For example, when silica sols are used todelusterize various objects, the flattening out elfect of the silica solis a direct proportionate function of the average particle diameter ofthe silica contained therein. Many other processes call for the use ofsuch large particle size silica in the form of sols, and recentlyefforts have been redoubled to achieve such large particle size silicasols.

It is evident to one skilled in the art that due to the inherentcharacteristics of silica, it is an extremely difficult task to producesilica sols which are stable over long periods of time, are concentratedsufficiently to meet practical demands, and yet, additionally, containsilica particles of sufiicient size to meet the required highperformance values of many treatment processes such as delusterizing andthe like.

To some extent the problems of stability and concentrat1on of silicasols have been overcome, although considerable time and efforts arestill being directed toward these ends. Efforts, however, to producerelatively large silica particles existing in the form of a uniform solwhich has the concurrent properties of stability and concentration havemet with little or no success. It is important to many industriesemploying these sols that the silica particles be kept in a discretestate, that is, in the form of non-agglomerated, spherical particles.Only when the silica particles are in such a form are the free silanolgroups on the surface of the particles completely free to react orperform some coactive function according to the particular use for whichthey are derived. When the properties of concentration, stability andlarge particle size are all desired in one product with the further pro-VlSO that the silica particles remain in a discrete, spherical, nOnaggregated form, the problems attendant to the production of such solsare of considerable magnitude. Heretofore, no process is known to havebeen devised whereby stable, relatively concentrated silica solscontaining large, spherical and discrete silica particles have beenproduced. Efforts toward this end usually result in dilute sols orsilica sols which are unstable for the requisite practical amounts oftime. Again, many of the same processes involving production ofconcentrated and/or stable silica products have the disadvantage thatonly a minor percentage of large silica particles are produced andresults from the desired use are unsatisfactory. On the other hand,efforts to increase the silica particle size of known silica sols haveonly resulted in substantial agglomeration of the silica through silanolcondensationtype reactions. These agglomerates of silica, as mentionedbefore, are often useless in many processes. These nonspherical orirregular agglomerates are joinned by siloxane bonding and are not onlyundesirable because of size and irregularity, but also have the addeddisadvantage of low required activity in additive treatments. In short,while these agglomerated silica particles measure above say, 40 or 50millimicrons as determined by supercentrifuging technique or some typeof light refraction method, the ultimate size of the component silicaparticles is well below this range as determined from measurement inelectron microscope photographs. These small particles have been merelyjoined into tight network by siloxane bonding to produce the aggregatesthereof with the aforementioned deficiencies.

In the current work toward achievement of a concentrated stable silicasol containing silica of relatively large weight-average particlediameter, many of the prior art techniques were thoroughly investigatedand subsequently discarded in view of their inadequacies with regard toall or several of the desired properties.

One of the earlier disclosed methods of building up the size of silicaparticles is discussed in Bechtold and Snyder, US. Patent 2,574,902.Following the techniques outlined in this prior art disclosure, thefollowing run was made: 40 B. commercial sodium silicate with a SiO :NaO ratio of 3.25:1, containing 28% SiO was diluted to about 4% Si0 withdeionized water and passed through a bed, 1% inch diameter x 15 in. ofNalcite HCR, a sulfonated styrene divinylbenzene copolymer cationexchange resin in the hydrogen form which had been backwashed and wellrinsed. Through this ion exchange technique the sodium silicate wasconverted to silicic acid or what is commonly known as active silica.900 ml. of this acid sol with a pH of 2.9 measured with a Beckman ModelG pH meter, glass electrode vs. calomel, calibrated against pH 7 buffer,were adjusted with 14 ml. of 1 N NaOH to give a sol with a SiO :Na Oratio of 85:1 and a pH of 7.2. 425 ml. of this sol were charged to a 500mL, 3-necked, round-bottomed flask equipped with stirrer, calibratedaddition funnel, and vapor take-off with removable stopper for samplewithdrawal from or material addition to the flask, and external heater.This reaction flask was heated and refluxed under atmospheric pressurefor 1 hour to form a heel. The alkalized $01 was then fed into thereaction flask so as to maintain constant volume during removal of waterby distillation. 300 ml. of 4% SiO feed and 1900 ml. of 3.8% SiO feed,all alkalized to a SiO :Na O ratio of 85: 1, were introduced over aperiod of 9 /2 hours, at which point the sol contained 36.5% SiO at a pHof 9.33 with a viscosity of 14.3 centipoises as measured on a BrookfieldModel LVF Viscometer. The sol so produced was turbid and hazy, with aturbidity index of 0.150 cm.- at 10% SiO The Si concentrations in theacid sol as well as in all other sol samples in this run and those runsdescribed hereinafter, were determined from specific gravitymeasurements.

During the above run, 25 ml. samples for analyses were pipetted from thereaction flask after stopping the feed and continuing heat input forabout minutes to effect complete reaction of the active silica. Portionsof these samples were titrated to determine specific surface area asdescribed in Analytical Chemistry, vol. 29, p. 1981 ff.

The number-average particle diameters were obtained from these specificsurface area determinations by the relationship D=3000/S, where D is theaverage particle diameter in millimicrons and S is the specific surfacearea in M /g. SiO Constant volume was maintained in the reaction flask,by adjusting the feed rate between sample removals; a new volume wasestablished after each sample removal so that particle concentration wasnot changed.

Table I describes the results of the various titrated samples Withdrawnduring the reaction at the various silica concentration levels in theheel. It must be noted that the maximum number-average silica particlediameter that was achieved is 12.7 millimicrons. While, of course, somesilica particles occur as larger size particles throughout the colloidalparticle range, the proportion of these as compared to the bulk of thesilica particles is small.

The final sol product from the Bechtold and Snyder process was alsomeasured to determine its maximum weight-average particle diameter bymeans of turbidity studies. The procedure used to measure theweight-average particle diameter was that outlined in Kolloid-Zeitschrift, vol. 173, No. 1, pages 38-48 (1960). These measurementswere made with a Klett-Summerson Photoelectric Colorimeter using a 1.25cm. cylindrical cell and a 400 m. filter. Using this technique the finalsol product was found to have a maximum weight-average particle diameterof 24.4 millimicrons. In other words, whether speaking in terms of anumber-average or a weight-average, the average particle diameter ofthese silica particles are well below 45 millimicrons in size. Furtherparticle build-up was impossible due to gellation or precipitation ofthe solid silica particle phase.

TABLE I Ml. feed Time, pH Percent Cone, D, mu

added hours SiOz g. SiOg/ml.

While the sols produced by the Bechtold and Snyder method aresufliciently stable for practical periods of time, their inadequacieswith regard to relatively low SiO concentration and low average particlediameter are apparent.

' Rule in US. Patent 2,577,484, describes an improved method overBechtold and Snyder silica particle build-up by again using a feed-heeltechnique. The following is a typical run made according to the Rulemethod. An acid sol of the type described above, with a pH of 2.8, waspassed through Nalcite WBR (aminated styrene divinylbenzene copolymerWeak base resin) in the free base fonn to give a deionized active silicawith a pH of 3.25 containing 3.7% SiO To 425 ml. of this sol was added6.65 ml. of l N NaOH to produce an alkalized sol with a pH of 6.33 and aSiO :Na O ratio of 150:1. This sol was then refluxed for minutes. The pHof the resultant heel was 7.84 at 25 C. The heel was heated to boilingand the addition of deionized acid sol was started. Sufficient base wasalso added from time to time to maintain the SiO :Na O ratio of :1. Foreach 200 ml. of feed added there was also added 1.2 ml. of 1 N NaOH. Thefeed and boil-01f were continued for 11 hours, at which time the solcontained 31.8% S10 at pH of 8.9 with a viscosity of 129 cps. After thistime a scale-like precipitate had formed in the bottom of the flaskindicating that further concentration was impossible. A total of 2700ml. of feed and 16 ml. of 1 N NaOH were added. At appropriate intervals,test samples were withdrawn from the reaction. The finished sol wasturbid and quite cloudy with a turbidity index of 0.211 cm.- at 10%S102.

Table 11 below shows the physical characteristics of the various solsamples withdrawn during the specific The weight-average particlediameter of the final sample as determined by the aforementionedturbidity technique was 31.3 millimicrons. The same inadequaciesdiscussed above with regard to the Bechtold and Snyder process are alsoevident here. Again, the maximum average particle diameter that may bereached whether speaking in terms of a number or weight average is below45 millimicrons. Even the most concentrated sol samples had an averageparticle diameter under this figure. Moreover, it was virtuallyimpossible to concentrate the sol containing the largest particles atthe end of the run, to a point much above 35% silica concentration.Toward the end of the experimental run substantial amounts of scalelikeprecipitate had formed and further concentration only resulted inincreased silica precipitation out of the continuous liquid aqueousphase.

Another method of effecting silica particle build-up is revealed inReuter et al., US. Patent 2,929,790. The method disclosed therein wasfollowed according to the following general procedure: Two liters ofacid sol produced as described above were adjusted to a pH of 8.60 usingthe same commercial sodium silicate as was used in production of theacid sol. 1350 ml. of this alkalized sol with a SiO content of 4.4% werecharged to a 2 liter flask and evaporated at atmospheric pressure to 475ml. This heel contained 12.6% SiO at a pH of 10.3. 425 ml. of this heelwere then charged to a reaction flask and brought to a boil whilestirring vigorously. Addition of acid sol, containing 3.9% SiO at a pHof 2.85 was started. During the addition of the remainder of the solconstant volume was maintained. 2400 ml. of feed were added during 10%hours at which point the sol contained 35.5% SiO The finished sol had apH if 9.02 and had a viscosity of 5.2 cps. The sol was turbid, but notcloudy with a turbidity index of 0.058 cm. at 10% SiO Physical data fromthe samples collected during this run is presented below in Table III.

The weight-average particle diameter of the final concentrated productwas also determined and found to be As in the previous two experimentalruns, this experiment also demonstrates that the achievement of a silicasol containing relatively large average diameter particles of silica wasnot available. As in the above discussed prior art disclosures, theproducts produced by this method may well contain silica particleshaving various diameters over a wide range of sizes. Again, however,whether a determination is made on the basis of a weight-average silicaparticle diameter or a numerical average thereof, results show that suchan average diameter falls below 45 millimicrons in size.

Broge et al., US. Patent 2,680,721 also describes a method of increasingthe size of unaggregated silica particles. In this process the same typeof acid sol as described above is alkalized, then pumped under highpressure through an extremely long stainless steel pipe immersed in amolten salt bath or some other type of appropriate container. Thepressure in this process must be maintained at a relatively high leveland the sol is heated at tempreatures from 160 to 300 C. While largeparticle size colloidal silica is produced, the process has the inherentdisadvantage that only relatively dilute concentrations of silica may beproduced, for example, around 3% silica. Concentrated products above 35%silica content are unable to be synthesized using the method of thisreference. Moreover, the expense of the process equipment involving hightemperature and high pressure requirements, and the various stepsinvolved make the process unattractive from this viewpoint. Thus, byfollowing the method of Broge et al., the achievement of stable highlyconcentrated silica sol products has been sacrificed to achieve silicaparticle sizes in excess of 45 millimicrons.

Much work has been done by Alexander and Her, with regard todetermination of particle sizes in colloidal silica. In their articlefrom the Journal of Physical Chemistry, vol. 57, p. 932, they haveslightly modified the Bechtold and Snyder method described above for usein their investigation. By continuously withdrawing samples, 51 in all,during the run, the authors were able to analyze the fractions forparticle size distribution using electron micrographs. While their workwas done primarily to show the difference obtained between adetermination of weightaverage particle diameter, the number averagediameter and the surface average diameter, of silica particles theautors additionally prove that only in the final few fractions wererelatively large silica particles produced. Most of the fractions showedthat the vast percentage of particles were well below 45 millimicrons insize.

In the above cited work, Alexander and Her show the addition ofalkalized acid sol, containing 2.4% silica, to a commercially availablealkaline sol containing approximately 30% silica. By a carefullycontrolled combination of continuous product removal and distillation ofwater, the silica content of the sol in their evaporator was maintainedat 30%. As evidenced by analyses of several sample fractions removedduring the experiment, the average diameter of the silica particlesincreased to a final value of about 60 millimicrons. During theexperiment, no concentration of silica in the evaporator was achieved,and, indeed, it is very diflicult to achieve concentrations much above30% Si0 by the Bechtold and Snyder method without causing gelation ofthe silica or a drastic increase in the sol viscosity resulting inunusuable produtcs. The removal from the evaporator of significantamounts of the product sol, as practiced by Alexander and Iler, would,of course, be an unattractive process from an industrial viewpoint, asthe yield of silica in the final product would be a small fraction ofthe total silica used in the process. Concentrated large particle silicasols above about 35% silica concentration could not be produced by apractice of this technique.

In view of the above discussed desiderata, it is evident that the priorart techniques all fail with regard to at least one or more of thedesired properties of silica concentration, stability and/or largeaverage particle diameter of colloidal silica. It would be a substantialimprovement in the art if stable silica sols of at least 35% silicacontent and having average silica particle diameters of above 45millimicrons could be produced in a simple, economical, single-stepprocess.

It therefore becomes an object of the invention to produce concentratedstable silica sols having average particle diameters in excess of 45millimicrons.

Another object is to produce these same stable concentrated largeparticle size silica sols by a simple singlestep process.

A specific object of the invention is to produce silica sols havingwater as the continuous phase and containing 3560% by weight of uniform,non-aggregated silica particles having a weight-average particlediameter from about 45 to about millimicrons.

A still further object is to produce concentrated stable silica solscontaining large silica particles by the process of treating a silicasol in such a manner that the starting silica particles are uniformlyincreased in diameter to from 2.5 to 4.0 times the original diameter,such uniform build-up being effected according to a predetermined silicaaddition rate formula.

A still further object is to provide stable concentrated silica solscontaining large, spherical, uniform, non-aggregated discrete silicaparticles with an average diameter of at least 45 millimicrons whichhave particular use in such processes as delusterizing and the like.

Other objects will appear hereinafter.

In accordance with the invention it has been found that concentratedstable silica sols containing at least 35 by weight of large, spherical,uniform, non-aggregated silica particles having a weight-averageparticle diameter of from about 45 to about millimicrons may be preparedby a novel process. In broad terms this process comprises the steps ofadding an acidic silica sol containing silica particles with an averagemolecular weight of less than 90,000 to a dilute aqueous alkaline silicasol containing less than 3.5% by weight of solid silica particles havinga weightaverage diameter of from about 10 to about 30 millimicrons. Thisaddition of acid sol to alkaline sol is maintained, while continuouslyevaporating the liquid aqueous phase of the alkaline sol at atmosphericconditions, according to the following rate formula:

where F is the maxmium feed rate at anytime of the acidic silica sol ingrams of silica contained therein per milliliter of alkaline sol perhour, k is a constant with a value of 5X10 when the temperature of thereaction is about 100 C., C, is the silica concentration of the alkalinesilica sol in grams per ml. at any time, C is the initial silicaconcentration of the alkaline silica sol, and S is the initial specificsurface area of the silica of the alkaline sol in square meters per gramof silica, all silica contents being expressed as SiO If the addition ismaintained according to the above rate formula and the pH of thealkaline sol is maintained over 7.0 units, the weight-average silicaparticle diameter of the starting silica sol may be uniformly increaseduntil sol products having a silica particle diameter of 45-100millimicrons are achieved. Preferably the starting silica particlediameter is typically increased from 2.5 to 4.0 times according to theinvention. The final products exhibit this uniform increase of particlediameter since they themselves, are spherical and uniform in appearancewith substantially no agglomeration having taken place during theprocess.

The term weight-averag particle diameter is defined as the diameter of asingle silica particle which has an average molecular weightrepresentative of the total mass of silica in the sol.

PROCESS OF THE INVENTION Starting alkaline sol As generally discussedabove, any alkaline silica sol containing silica particles having aweight-average diameter of from about 10 to about 30 millimicrons may besuitable for use as a starting sol in the process of the instantinvention. The silica particles must be discrete entities and sphericalin form having at least the weight-average diameter above. In otherwords, agglomerated or reticulated silica particles in the form of solsthereof are not contemplated for use in the invention. The silicaparticles of these alkaline sols have specific surface areas which mayvary from 45 to 300 M /g. SiO

As has been described above, much of the prior art has been concernedwith production of silica sols containing silica particles of less thanabout 45 millimicrons in average diameter. These prior art finished solsare admirably suited as reactants in the instant invention. It isessential, however, that the weight-average silica particle diameter beat least 10 millimicrons. More preferably the weightaverage particlediameter of the starting sol may range from 15 to 25 millimicrons. Sols,as produced by the methods: Bechtold et al., U.S. Patent 2,574,902;Rule, U.S. Patent 2,577,484; Reuter et al., U.S. Patent 2,929,790, maybe used as my starting alkaline sols. Other typical starting silica solsmay be made by resort to U.S. Patents 2,601,235, 2,680,721 and2,929,790.

In addition to the alkaline silica sols described in the abovereferences, resort may be had to those sols generally characterized asdeionized. These sols may be either completely deionized by a mixedresin bcd treatment or may be merely decationized by the appropriateresin. However, when using these type sols it is necessary to addthereto suflicient alkalinity by use of alkali metal hydroxides oralkali silicates so that the pH is adjusted to at least 7.0 units.

Again, the starting silica sols employed in the invention may containany type of hydrophilic liquid carrier without departing from the spiritof the invention. However, for practical considerations, it is preferredthat the liquid phase of the starting alkaline silica sol be water.

Typical commercially available silica sols which may be used as thestarting alkaline sol materials are those silica sols which are sold bythe Nalco Chemical Company under the trademark Nalcoag. The physical andchemical properties of these types of silica sols are set forth in TableIV.

TABLE IV So! I Sol II Sol III Percent. colloidal silica as S102 30-3135-36 40-50 pH 10. 2:1:0. 2 8. 6:1:0. l 9. :0. 1 Viscosity at 77 F. cp 55 20-30 Specific gravity at 68 F 1. 205-1. 21 1. 248-1. 255 1. 385-1.394 Surface area, M per gram S102 100-270 135-100 120-150 Averageparticle size, millimicrons 11-16 16-22 20-25 Density, per gel. at 68 F10.0 10. 5 11. 6 Freezing point 32 32 32 N e20 percent 0. 40 0. 10 0. 30

As before mentioned, almost any type of alkaline silica sol may be usedfor the invention as starting material, which has a weight-averageparticle diameter of from 10 to 30 millimicrons in size. However, morepreferable starting alkaline silica sols are those which contain silicaparticles having a weight-average particle diameter of from 15 to 25millimicrons.

The pH of the starting alkaline silica sol must be maintained above 7.0during the whole of reaction. If the alkaline sol during the process isallowed to become more acid or the pH is allowed to fall below 7.0 unitsthe danger of gelation is greatly increased. In view of this, it ispreferred that the pH of the starting alkaline sol range between 7.5 and11.0 and more preferably between 8.5 and 10.0.

The problem of maintaining the proper alkalinity can be overcome by twoapproaches. In one embodiment, the pH can be periodically adjusted byaddition of the appropriate basic substance at various intervals duringthe reaction. If the alkalinity is periodically adjusted, it ispreferred that the pH of the alkaline silica sol be maintained withinthe range of 7.0 to 10.0. Any appropriate substance may be used which isa sufliciently strong base so that relatively small amounts arenecessary to maintain the proper pH and which does not form an insolublesilicate. However, such substances as alkali metal hydroxides and alkalimetal silicates as sodium hydroxide and sodium silicate may be used tobest advantage.

In another more preferred embodiment, suflicient alkalinity isintroduced into the starting alkaline silica sol, before any acid sol isadded thereto, so that during the course of the reaction no furtheraddition of alkaline substance is necessary. If such is desired, the pHis properly adjusted by putting in basic materials such as alkali metalhydroxides and alkali silicates, so that it ranges between 9.5 and 11.5units. More preferably the pH is adjusted before any acid sol is addedso that the initial pH is l0.011.0.

Another very important consideration is the concentration of thealkaline silica sol. It has been affirmatively determined thatregardless of the particular source of alkaline sol used, it must berendered dilute either by addition of water or some hydrophilic organicsubstance, in order to achieve the desired uniform coating reaction.Preferably the starting alkaline silica sol is aqueous and has beendiluted with water below about 3.5% concentration of solids by weight.More preferably, however, the silica solid content of the startingsilica S01 is adjusted between about 0.5 and 2.5%.

Acid sol The particular acid sols used as the active coating reagentsmay be produced by a wide variety of methods. All of these particularsols have average molecular weights below about 90,000. More preferablythese acid sols contain silica particles having an average molecularweight of from about 1000 to 46,000. The pH of these acid sols is below515 and more preferably they lie within the range of 2.5 and 3.5.

One method of preparing such acid sols is to neutralize water glass witha mineral acid. In using this method to form the acid silica sols it isnecessary, however, to remove the major portion of the salts formed byneutralization reaction. This may be accomplished by dialysis orelectrodialysis, but these procedures are not not adaptable to largescale economic production. An improved method for preparing acid solshas been described in Bird, U.S. Patent 2,244,325. By utilizing theteachings of this patent the preferred starting acid sols are produced.According to the Bird method a water glass (alkaline silicate) solutionis passed through a column of cation exchange material in the hydrogenform whereby the alkali metal in the water glass is exchanged forhydrogen and the resultant product is an acid silica sol of unusualpurity. Generally, the pH of the sols so produced lie within the rangeof 2.04.0. In addition, the average molecular weight of the silicaparticles is well below 90,000.

Other acid sols suitable for use in the invention may be prepared by avariation of the Bird method described above. In this embodiment theeflluent from the Bird process may then be further treated by passing itthrough a weak base resin in the free base form. The resultant productis then substantially stripped of any ions and is generally known asdeionized. Still another variation of the technique is to employ a mixedresin bed, that is, a bed containing a weak base resin in the free baseform and a strong acid resin in the hydrogen form whereby the silicicacid sol is formed simultaneously with exchange of its companion ions toproduce a substantially deionized polysilicic acid sol.

While the above described methods are preferable to produce the startingacid sol, it must be understood that any appropriate method forproducing an acid sol of a requisite molecular weight and pH may also beused. For example, minute amounts of the stabilizer such as alkali metalhydroxide may be used without departing from the scope of the inventionas long as the pH is not raised above the operative limits describedabove.

Rate of addition A critical aspect of the invention is the properadjustment of addition of acid sol with the above describedcharacteristics, to the aforementioned particular alkaline silica sol.It has been determined that this rate of addition may be appropriatelyfixed according to the following rate formula, when the reaction iscarried out at the boiling point of the aqueous sol under ambientconditions:

where F is the maximum feed rate at any time of the acid silica so] ingrams of silica per milliliter of alkaline sol hours, k is a constantwith a value of X under the reaction conditions just stated, C, is thesilica concentration in grams of silica per ml. of the alkaline silicasol at any time, C is the initial silica concentration of the alkalinesilica sol, S is the initial specific surface area of the silicaalkaline sol in square meters per grams of SiO all silica contents beingexpressed as SiO by weight.

The above feed rate equation was derived as follows:

It has been found that the active silica concentration from the acid solwhen added to the alkaline sol is constant in the reaction mixture.Therefore, the feed rate is directly proportional to the rate ofdisappearance of active silica in the acid sol. It has also been foundthat all the active silica of the acid sol combines with the seed silicaparticles of the alkaline sol below a certain active silicaconcentration in the reaction system. Therefore, the maximum feed ratefor the desired particle growth which will define the active silicaconcentration of the acid s01 may be obtained as follows:

Equation I F: kS'

where S is the specific surface area in M /grams SiO of the alkaline soland C is the silica concentration in grams SiO /ml. of the alkaline sol.

SiO

(C'=percent 100 Combining Equations I and II, F =kSC. The initial feedrate then is given by:

Equation III density of sol) Fo=ksoco 10 where S and C describe theinitial specific surface area and concentration of the alkaline sol andF is the initial feed rate.

The feed rate at any time, t, is therefore given by the followingequation:

Equation F =kS C In order to determine S, the surface area per gram ofSiO the following derivation is made:

The volume of an average particle is given, by the volume of a sphere,that is,

1rd ii where a is diameter of an average particle. The weight of thisparticle is given by 1rd p where p is the density in grams/ml. Thenumber of particles, n, in one gram is then The surface area of anaverage particle is given by S=1rd Also, s=S/n therefore, S=ns.Concluding 6 6 (ash m In ideal particle growth, all acid silica feedwill accrete on particles already present, with no new particles formed.In addition, it is assumed there is no agglomeration of particles.

The number of particles in one ml. of sol containing C grams/ml. ofsilica is given by the relation between initial and final particlediameters and silica concentrations in an ideally grown sol.

Therefore, the specific surface area per gram of silica particle at anytime is:

Equation V 4 a 1/3 stmdo t) By inserting Equation V in Equation IV thefollowing equation is derived: Equation VI F k c usg z/s Finally, bysubstituting for A/d the final rate formula is determined as follows:

Equation VII F kS C C The value of k is then determined experimentallyfrom the maximum feed rate which gives the desired growth and from theknown properties of the alkaline sol. These experimental values are thensubstituted in Equation III to yield the value for k. For aqueousalkaline sols wherein the boil-off is affected at the boiling point ofwater under atmospheric conditions, k has been determined as It isimportant to follow a rate of addition of acid sol according to theabove formula for two reasons. First, only by following such a definedrate is one able to inhibit the formation of new silica sites in thealkaline silica sol which may occur by polymerization of the acid solwith itself upon addition. In other words, it is necessary to keep theaddition rate of the acid sol sufiiciently low so that it, in reality,polymerizes upon the surface of the silica particles of the alkalinesilica sol rather than condenses with itself in the reaction mixture toform undesirable particles of relatively low weight-average particlediameter. In effect, if the acid sol is allowed to polymerize itself,small polymeric seeds are formed which act as acceptors of furtheramounts of the poymerizable acid sol added thereto. This is to bespecifically avoided.

Another very important reason for adhering closely to the terms of therate formula is to promote uniformity of coating upon the silicaparticles of the alkaline sol existing in the reaction mixture. If theterms of the rate formula are followed as described, the silica of theacid sol is polymerized and coated upon the silica particles of thealkaine sol in a uniform manner so as to have the requisite sphericalcharacter in addition to the increased weight-average particle diameterof the ranges described above. An uncontrolled or random rate ofaddition results not only in new polymeric sites which promote lowweight-average diameters but also would result in nonspherical shapedparticles. To achieve particle uniformity and spherical shape it isessential that the silica particles of the alkaline sol be built up orincreased in diameter continuously. This is only effectual by followinga rate of addition of acid sol defined by the above formula.

By following the defined rate of addition according to theabove-described formula it has been determined that the weight-averageparticle diameter of the silica particle contained in the alkalinesilica sol have been increased from 2.5 to 4.0 times in size. Morepreferably the particle sizes are increased 2.5 to 3.5 times theoriginal starting diameter.

The total time addition of acid sol may range from 8 to 48 hours inlength. It has been determined that under the most favorable processconditions the time of addition may vary from 12 to 30 hours and mostpreferably from 18 to 28 hours. During this time, the amount of silicagenerally added from the acid so] is 10-30 parts per 1 part of silicacontained in the starting alkaline silica sol. However, this figure mayvary according to all the variables defined in the rate formula and theparticular process equipment employed.

If an acid sol is added to the alkaline silica sol at the appropriatedefined rate, the silica particles contained in the reaction mass, thatis, the silica particles of the alkaline silica sol, are increased insize from 1-5 millimicrons per hour of reaction time. More preferablythe weightaverage particle diameter of the silica particles of thealkaline silica sol are uniformly increased 1 to 3 millimicrons perhour.

As mentioned above, it is essential that the alkaline silica sol whoseparticles are to be built up in magnitude must be maintained at a pH ofat least 7.0 and more preferably above 7.2. Whether or not suificientalkalinity is added in the beginning as a one-step addition or added inappropriate increments during the whole of reaction, it is necessarythat the pH be kept on the alkaline side. If such is not done, gelationand/or uneconomical precipi tation will occur, and products with therequisite uniformity of size and magnitude of weight-average particlediameter cannot be produced.

During the course of the reaction the temperature of the reactionmixture is kept at about the normal boiling point of the aqueous liquidcarrier of the alkaline silica sol or slightly above which, of course,for water is about 100 C. Thus, in effect, while all the particlediameters are being continuously built up, the additional effect ofconcentration is also taking place so that the final large particle sizesilica sol is concentrated to a point above 35% solids by weight of SiOThis evaporation of the liquid phase of the alkaline silica sol, whetherit be of water or some appropriate hydrophilic organic reagent, may beaccomplished at atmospheric pressure, a pressure somewhat aboveatmospheric, or under an appropriate vacuum. If a vacuum technique isemployed, of course, the boiling temperature of the continuous aqueousliquid phase is lower than at atmospheric conditions and con verselyhigher if pressure is employed. However, it is preferred because ofpractical considerations with regard to the process equipment involved,that the evaporation be effected at ambient pressures.

The constant, k, in the above rate formula will assume different valuesif the reaction is run under pressure or vacuum, or if the continuousphase of the sol has a normal boiling point substantially different fromthat of water. The proper k constant applicable to the Particularhydrophilic liquid employed-and conditions of evaporation may bedetermined by proper experimentation by one skilled in the art.

Compositions of the invention By following the process techniquesoutlined above in any of their embodiments, concentrated stable alkalinesilica sols containing at least 35% by weight, expressed as SiO oflarge, spherical, uniform, non-aggregated silica may be made. These solscontain particles having a weight-average particle diameter of fromabout 45 to about millimicrons in size. The more preferred products arethose with silica particle diameters falling within the range of 45-80millimicrons and most preferably between 45 and 75 millimicrons. Thespecific surface area of these products ranges from 35 to M /g. ofSiO;,, and preferably from 40 to 100 M g. SiO A hydrophilic liquidcarrier of these sol products may be any hydrophilic organic substancesuch as lower alkyl alcohols or water, with the most preferablecontinuous liquid phase being an aqueous phase.

Further investigation of the sol products of the invention show thatthey all have viscosities of less than 10 cps. at 25 C. measured at 50%SiO with the majority of the products having a viscosity within therange of 5 to 10. Also, the sol products have conductivities less than5000 micromhos with the bulk of the compositions having conductivitieswithin the range of 3000-5000 micromhos measured at 50% SiO If theaforementioned process is run under the most preferred conditions, thatis, the liquid carriers of both the alkaline and acid silica sols arewater, then the final product will contain water as a continuous phase.This aqueous phase may be exchanged with a hydrophilic liquid by a widevariety of published techniques to yield a concentrated final producthaving a hydrophilic organic liquid phase and large silica particles ofalready defined size existing as the solid dispersed phase of the solproduct.

The large-particle silica sols so produced are all alkaline in naturewith the pH ranging between 7.0 and 10.0 units. However, when the solsare produced by the most preferred process embodiments, their pH fallsbetween 7.5 and 10.0 units.

As mentioned before, the large silica particles so produced aresubstantially uniform and spherical in shape. Moreover, these particlesexist as discrete entities, that is to say, they exist as non-aggregatedparticles having a plurality of SiOH groups on their surfaces. The mostpreferred products contain 35-60% by weight of these uniform, large,discrete silica particles. Even more concentrated products arepossibleby folliwing the practice of the invention. Heretofore, silica particlesof such definition and in such concentrations have been unavailable fromprior art techniques.

In order to further prove impossibility of forming large, discrete,spherical silica particles having a weight-average particle diameter offrom 45 to 100 millimicrons via prior art conventional one-stepheel-feed processes as set out in previously discussed patents andothers, the following experiments were carried out.

From the above theory it was developed that where d =particle diameterat time t, C =concentration (g./cc.) of sol at time t, and d and C arethe initial conditions. It appears then that at least theoreticallylarge particle size silica sols can be made by simply controllinginitial heel particle size and concentration. From the equation, it isobvious that the more dilute the heel or the larger the heel particlesize, the larger the particle size that can be achieved in the final solin any desired concentration.

However, certain limiting considerations come into play. First of all,conventional heels are generally prepared by alkalizing acid sols fromthe method described in Bird, U.S. Patent 2,244,325. The maximum silicaparticle size in these heels is about millimicrons and generally rangesfrom about 3 to about 5 millimicrons in particle size. From the aboveequation and employing initial heel sols of this particle size, it wouldrequire initial heel concentrations of less than 0.1% silica solids totheoretically achieve via a one-step process a sol having a finalparticle size greater than 45 millimicrons. Yet, at such dilute silicaconcentrations the solubility of amorphous silica, which ranges from0.015% at room temperature to 0.04% at 100 C. becomes relatively greatcompared to total amount of silica present. This solubility then becomesa limiting factor in heel development. As an average, then, the lowestsilica concentration in a heel sol which can be reached without merelysolubilizing the silaca, and thereby completely preventing particlegrowth is approximately 0.02%

In order to verify the above, a heel was prepared by alkalizing an acidsol obtained from the Bird process, U.S. Patent 2,244,325, to a pH of7.0. Specifically, 200 ml. of this sol was placed in a 500 ml., 3-neckflask. Acid silica sol was fed into the alkaline heel at a constant potvolume and within the rate discovered here to be critical. After 23hours total time, a 50% sol was reached. The product had a particlediameter of 44.2 millimicrons. However these, particles were not uniformin size despite maintaining the feed rate at such a level which was slowenough to prevent nucleation of new particles. In a number of otherexperiments, the silica concentration was increased. In no instance didany sol even reach the above figure much less a maximum particlediameter of 45 millimicrons. This is true even though the acid sol feedrate described herein was closely followed. In yet other experiments,the pH of the alkaline heel sol was increased. This resulted in silicaparticle size diameters even lower than those achieved in the aboveexperiments.

Thus, it appears that regardless how one varies all possible processvariables in the one-step, heel-feed prior art process, it is impossibleto achieve growth of silica particles greater than about 45millimicrons, and specifically within the range of 45100 millimicrons.This is true even when one works within the feed rates described hereinand found also to be critical. Thus, it is apparent that as an essentialnecessity, in addition to closely following acid feed rates set outherein, the starting alkaline heel silica sol must have a weight-averagediameter of at least millimicrons to achieve the desired large particlesilica sols. Such concept was heretofore undiscovered in the art.

EXAMPLES The invention will be better understood by reference to thefollowing illustrative examples:

14 Example I 33.5 grams of an aqueous alkaline silica sol containing 50%SiO by weight with a pH of 9.0 and a weight-average particle diameter of21.3 millimicrons were diluted to 800 ml. with deionized water.Thisalkaline silica sol containing 2.3% SiO at a pH of 9.4 was chargedto a 1 liter 3-necked flask equipped with a stirrer, calibrated droppingfunnel and condenser with an adapted distillate take-off. The contentsof the flask were heated to boiling and dilute acid sol (polysilicicacid) containing 4% SiO was added. This acid sol had been previouslyprepared by passing diluted sodium silicate to a strong acid ionexchange resin in the hydrogen form according to the general methodoutlined by Bird, U.S. Patent 2,244,325. The acid sol was added throughthe dropping funnel to maintain constant volume in the flask and replacethe constantly distilling water which was continuously removed. Theheating rate was controlled so as to maintain the boil-off rate at about350 ml./hr. The pH in the flask was maintained above 8.0 by requisiteadditions of 1 ml. portions of 40 B. sodium silicate (SiO :Na O=3.25 :1)until the SiO content in the flask reaches 15%. After the concentrationhad reached this point, the pH was maintained between 7.5 and 8.0 byaddition of 0.5 ml. portions of sodium silicate of the type describedabove. The addition of acid sol and boiloff continued at theaforementioned rate until 9.1 liters of active silica solution had beenadded to the flask, at which point the reaction was stopped. Thefinished sol product contained 37.4% SiO at a pH of 8.05 at 25 C. and aSiO :Na O ratio of 650: 1.

The aforementioned addition of acid sol was performed an accordance withthe required rate as previously defined by the rate of addition formulaabove. The average particle diameter of the sol as determined bytitration for specific surface area was 48 millimicrons based on aspecific surface area of 63 M /g. This determination was made accordingto the technique outlined in Anal. Chem. 28, 1981 (1956). When dilutedto a solids concentration of 1.25% SiO by weight, the sol had an opticaldensity of 0.66 as measured in a /2 inch test tube against a deionized'water sample using a Kiett-Summerson Photoelectric Colorimeter Model900-3 with a No. 44 filter.

The Si0 concentrations were determined from specific gravitymeasurements. The pH was measured with a Beckman Mode G pH meter, glassvs. calomel electrodes, which had been calibrated against pH 7 bulfer.

Example II In this example an alkaline silica sol similar to the oneused in Example I was used with the exception that 16 -ml. of 40 B.sodium silicate (SiO :Na O=3.25 :1) were added to the alkaline solbefore heating. The rate of addition of acid sol again conformed to thatrequired and defined by the rate formula of the invention. This gave atotal silica concentration of 3.2% SiO by weight, and a pH of 10.8 andan optical density of 0.094 at 1.25 A total of 7.1 liters of acid solproduced as described in Example I above were added to the flask whilemaintamed a constant volume by distillation of the water phase at aratio of about 400 ml./hr. The final SiO :Na O ratio was :1 and thespecific surface area was 68 M /g., corresponding to an average particlediameter of 45 millimicrons.

Example III 30.5 grams of an aqueous 50% Si0 sol with a pH of 9.0 and anaverage particle diameter of 21.3 millimicrons were diluted to 425 ml.with water, giving an alkaline sol containing 3.5% SiO A 6% solution ofthe acid sol produced as described in Example I was fed into thealkaline sol which had been added to a 500 ml. flask. The flask wasagitated rapidly and water distilled therefrom to maintain constantvolume. The pH of the reaction moisture in the flask was controlled byaddition of 1 N NaOH in 1 ml. increments. After the silica concentrationreached 20% the pH was maintained between 7.0 and 7.5. After 2.95 litersof acid sol had been added at a rate satisfying the requirements of therate of addition formula, the alkaline sol in the reaction flaskcontained 44% SiO at a pH of 7.02. The sol in the reaction flask at thistime had a viscosity of less than 10 c.p.s. After adjusting the sol topH of 7.6, it was concentrated by evaporation to 54% SiO The finalproduct having this concentration had a pH of 7.47, an average particlediameter of 48 millimicrons, and a SiO :Na O ratio of 570:1. Storage for2 months at room temperature showed that the sol product was stableagainst any type of gelation for at least this period of time.

Example IV An alkaline silica sol was prepared which contained 50% SiOby weight and had an average particle diameter of 22.4 millimicrons. 690ml. of this 50% sol were added to 6 gallons of soft Water and 450 ml. of40 B. sodium silicate were then added to the mixture. This dilutealkaline sol was then charged to a submerged steel tube evaporator. Theviolent, almost explosive, boiling within the submerged tubes serves tovigorously agitate the entire contents of the evaporator, which is auseful device for obtaining a large heat transfer surface. A centrifugalpump recirculating the evaporator contents to an external line was usedin conjunction with the evaporator. Steam pressure was adjusted to givea boil-off rate of about 18 pounds of water per hour. Constant volumewas maintained by adding to the alkaline sol a 4.5% acid sol made asoutlined in Example I. This acid sol was fed into the recycle lineimmediately before the pump to insure adequate mixing. During theevaporation the recycle rate was about 0.7 gallon/min. The addition ofpolysilicic acid sol was continued for 26 hours at a rate of about 2.1gal./hr., which addition rate conformed to that defined by the aboverate formula. After this addition time, the sol in the evaporator had anSiO content of 35% with a pH of 8.8 and average particle diameter of 61millimicrons. At 1.25% SiO the sol had an optical density of 0.87.

Example V This sol product was prepared as generally out-lined inExample IV with the exception that the boil-ofi rate was maintained atabout 24 pounds/hr. for the first hours and 32 pounds/hr. until the runwas completed. The total time of addition was 14% hours. The finishedsol product contained 35.5% SiO and had a pH of 8.7. The averageparticle diameter of the finished sol was 63 millimicrons. When dilutedto 1.25% SiO the sol had an optical density of 0.84 when measured asgenerally outlined in Example I. 29.7 pounds of the sol as produced inExample IV were then added to 50 pounds of the sol of this examplecontained in the evaporator, while water was distilled oil? to maintaina constant volume. The resultant sol contained 50% SiO This productexhibited excellent stability against gelation.

Example VI A sol product was prepared as described in Example IV withthe exception that the boil-off rate was maintained at about 28 lbs. ofwater per hour throughout the course of the run and acid sol was fed tothe evaporator at a rate of approximately 3.3 gal./hr. The total time ofaddition was 18 hours. The finished sol product contained 35.0% SiO andhad a pH of 8.6. The average diameter of the silica particles in thefinished sol was 29 millimicrons. When diluted to 1.25% SiO the sol hadan optical density of 0.43. An electron micrograph of a diluted portionof the finished sol showed that, although there were some largeparticles, there was a great preponderance of smaller particles. Theformation of these smaller particles was the result of the higherinitial feed rate of the acid sol, and therefore higher active silicaconcentration in the evaporator, which permitted condensation of theacid sol particles with each other rather than reaction of the acid solwith the silica particles of the alkaline sol. This example demonstratesthe necessity of carrying out the silica particle build-up according toa rate defined by the formula above. If one exceeds, as in this example,the maximum limits of rate of addition of acid sol, the resultantproducts contain silica particles of relatively low particle diameter.This is due to formation of new seeds of silica in the alkaline silicasol which act as acceptors for the subsequently added acid silica sol,thus precluding the relatively larger, partially built-up silicaparticles from finally attaining their preferred large particle sizediameters.

Example VII 754 pounds of an alkaline sol containing 35% silica wereadded to a submerged tube, steel evaporator containing soft water togive a volume of about 1400 gallons. 25 gallons of 40 B. sodium silicatewere then added to the dilute sol. The diluted alkaline sol contained2.3% SiO before addition of the sodium silicate and the equivalent of3.2% SiO after the silicate addition and had an optical density of 0.126when measured according to the procedure of Example I. The alkaline solwas brought to a boil and an acid sol (4.3% SiO as produced according tothe procedure outline in Example I, was added at a rate of approximately400 gallons per hour for 12 hours. During this time water wascontinually distilled to maintain constant volume. After 12 hours theaddition and boil-off ratios were then increased so that 750 gallons perhour of Water was removed. After a total of 24 hours addition of acidsol, the boil-off and acid sol addition were stopped. The resultant solhad an average particle diameter of 73 millimicrons as determined by theaforementioned titration technique and an optical density of 0.920 asmeasured in Example I. The final solids content was 35.8%.

As readily seen by the foregoing examples, the products of the inventionas produced by the novel disclosed process all contain silica particleswithin the defined limits. These product sols are all relativelyconcentrated, that is, 35% by weight or above. Moreover, the largeparticles of silica contained therein are spherical, substantiallyuniform, and separate entities of 45 millimicrons or greater in particlediameter. Substantially no aggregation takes place during the process,thereby leaving the surface silanol groups free for subsequentmodification or reaction. The products when compared to those producedby prior art methods are substantially different with regard to theirweight-average particle diameter in particular and also materially varyin their properties of uniformity, non-aggregation, solidsconcentration, etc.

FIG. 1 is a graph of a typical large particle sol of the inventionshowing its gradual increase in weight-average particle diameter withincrease in concentration. The final sol product had a weight-averageparticle diameter of close to millimicrons. It is readily seen in thegraph that the particle growth is uniform and a straight line functionof the concentration of the alkaline sol, when acid sol is added theretogradually with simultaneous boil-off of the aqueous phase of thealkaline sol.

FIG. 2 of the drawing gives a visual comparison of the relative size ofproducts prepared in the instant invention as compared to preparationsby process of prior art disclosure. The smaller particles of Section Aare those silica particles prepared by a typical run according to theprocess outlined by Bechtold and Snyder, US. Patent 2,574,902. Afterthis typical run had been made, the sol products were prepared forphotomicrographic investigation and then so reproduced under amagnification of 65,000 The average particle size was 15 millimicrons.

Section B of FIG. 2 is a photomicrograph of a typical run according tothe method outlined in the instant invention. The sol product obtainedfrom this run had an average particle size of 62 millimicrons and wasthen magnified 55,000 The difference between the two sols so compared isclearly evident with regard to their silica particle diameters. Becauseof the drying and other manipulative techniques necessary to effect goodphotomicrographic analysis, the particles of both the prior art and theinstant invention appear agglomerated. This is illusory. The distinctcharacter of the particular silica entities is evidenced by the definitewhite border observable around each individual silica particle. Anagglomerate would appear as a large solid lump with no observableboundaries between component silica particles. Section C of FIG. 2 is aphotomicrograph of silica particles contained in a sol prepared by atypical run according to the process outlined in Renter, US. Patent2,929,790, mentioned above. This sol was prepared for photomicrographicexamination and photographed as shown in FIG. 2, Section C at amagnification of 60,000X. The average particle diameter was 25millimicrons. Again there is a very obvious difference between silicaparticle diameters of Section C as compared with Section B of FIG. 2.

The large, silica particles contained in the product sols of theinvention may be used for a wide variety of industrial and consumerprocesses. They are particularly adapted to delusterizing textiles andother materials and are admirably suited as anti-skid agents.

The products of the invention are also compatible with many hydrophilicorganic liquids and are capable of being incorporated into a widevariety of chemical products. When combined with other hydrophilicliquids or when used alone, the products may be utilized in the surfacemodification of plastics, rubber, textiles and the like. Anotherparticular desirable use would be the incorporation of the compositionsof the invention in non-gloss varnishes, to accomplish a fiattingeffect.

As indicated above, the compositions of the invention are of value inimproving the frictional characteristics of metal surfaces that move onewith respect to the other. This would apply to force fitted pinion gearsand shafts, or nuts and bolts having mating surfaces coated withcolloidal silica. The coefficient of friction between such parts ismeasurably increased due to the large particle size of the silicacontained in the sol products used in their production. The inventionis. hereby claimed as follows:

I claim:

1. The method of preparing concentrated aqueous stable silica solscontaining at least 35% by weight of large, spherical, uniform,non-aggregated silica particles having a weight-average particle of fromabout 45 to about 100 millimicrons which comprises the steps of addingan aqueous acid silica sol containing silica particles with an averagemolecular weight less than 90,000 to an aqueous alkaline silica solcontaining less than 3.5% by weight of silica particles having aweight-average diameter of from about 10 to about 30 millimicrons,maintaining said addition while continuously evaporating the aqueousliquid phase of said alkaline sol until the weight-average silicaparticle diameter of the starting alkaline sol is uniformly increased towithin said 45- 100 millimicron range, said addition being carried outaccording to the following rate formula:

where F is the maximum feed rate at any time of the acid silica sol ingrams of silica per milliliter of alkaline sol per hour, k is a constantwith a value of 5 X" when the liquid aqueous phase is evaporated underambient conditions, O, is the silica concentration of the alkalinesilica sol in grams of silica per milliliter at any time, C is theinitial silica concentration of the alkaline silica sol,

and S is the initial specific surface area of the silica of the alkalinesol in meters squared per gram of silica, all silica contents beingexpressed as SiO and maintaining the pH of the alkaline sol during thewhole of said addition at a value of at least 7.0 units.

2. The method of preparing concentrated aqueous stable silica solscontaining at least 35% by weight of large, spherical, uniform,non-aggregatedsilica particles having a weight-average particle of fromabout 45 to about millimicrons which comprises the steps of adding anaqueous acid silica sol containing silica particles with an averagemolecular weight less than 90,000 to an aqueous alkaline silica solcontaining less than 3.5% by weight of silica particles having aweight-average diameter of from about 10 to about 30 millimicrons,maintaining said addition while continuously evaporating the aqueousliquid phase of said alkaline sol until the weightaverage silicaparticle diameter of the starting alkaline sol is uniformly increasedfrom 2.5-4.0 times, said addition being carried out according to thefollowing rate formula:

where F is the maximum feed rate at any time of the acid silica sol ingrams of silica per milliliter of alkaline sol per hour, k is a constantwith a value of 5 10 when the liquid aqueous phase is evaporated underambient conditions, C is the silica concentration of the alkaline silicasol in grams of silica per milliliter at any time, C is the initialsilica concentration of the alkaline silica sol, and 8,, is the initialspecific surface area of the silica of the alkaline sol in meterssquared per gram of silica, all silica contents being expressed as SiOand maintaining the pH of the alkaline sol during the whole of saidaddition at a value of at least 7.0 units.

3. The method of claim 1 wherein the pH of the alkaline sol during thewhole of said addition is maintained above 7.0 units by adding in asingle addition to the starting alkaline sol sufficient alkalinity sothat the pH of said starting sol is from 9.511.5, said alkalinity beingproduced by addition thereto of a compound from the group consisting ofalkali metal hydroxide and alkali metal silicate.

4. The anethod of claim 2 wherein the weight-average silica particlediameter of the starting alkaline sol is increased frorn 2.5-3.5 times,said starting alkaline silica sol containing silica particles having aweight-average diameter of from about 15 to about 25 millimicrons.

5. The method of preparing concentrated aqueous stable silica solscontaining from about 35-60% by Weight of large, spherical, uniform,non-aggregated silica particles having a weight-average particlediameter of about 45 to about 70 millimicrons which comprises the stepsof adding an aqueous acid silica sol containing silica particles with anaverage molecular weight of less than 90,000 to an aqueous alkalinesilica sol containing from about 0.5 to about 3.5% by weight of silicaparticles having a weight-average diameter of from about 15 to about 25millimicrons, maintaining said addition while continuously evaporatingthe liquid aqueous phase of said alkaline sol until the weight-averagesilica particle diam eter of the starting alkaline so] is increased from2.5-3.5 times, said addition being carried out according to thefollowing rate formula:

where F is the maximum feed rate at any time of the acid silica sol ingrams of silica contained therein per milliliter of alkaline sol perhour, k is a constant with a value of 5 10 when the liquid aqueous phaseis evaporated under ambient conditions, C, is the silica concentrationin grams of silica per milliliter of the alkaline silica sol at anytime, C is the initial silica concentration of the alkaline silica sol,and S is the initial specific surface area of the silica of the alkalinesol in 19 20 meters square per gram of silica, all silica contents beingReferences Cited expressed as SiO and maintaining the pH of the alkalineUNITED STATES PATENTS sol during the whole of said addition above 7.2mm.

6. The method of claim 5 wherein the pH of the 2443512 6/1948 Powers atalkaline sol during the whole of said addition is main- 2,577,48512/1951 Ruletained above 72 units by adding in a single addition step 521929790 3/1960 Renter et sufiicient alkalinity to the starting silicaso] so that the pH of said starting sol may vary from about 9.5 to about11.5, said alkalinity being imparted thereto by a compound from thegroup consisting of alkali metal hydroxide 10 and alkali metal silicate.106286; 252-309 RICHARD D. LOVERING, Primary Examiner.

